This invention relates to compositions and processes for treating metal surfaces with acidic aqueous compositions to form conversion coatings on the metals; the conversion coatings provide excellent bases for subsequent painting. The invention is well suited to treating iron and steel, galvanized iron and steel, zinc and those of its alloys that contain at least 50 atomic percent zinc, and aluminum and its alloys that contain at least 50 atomic percent aluminum. Preferably the surface treated is predominantly zinciferous; most preferably the surface treated is galvanized steel.
U.S. Pat. No. 5,427,632 of Jun. 27, 1995 to Dolan (hereinafter usually denoted as “the '632 patent”) teaches that effective coatings of this type can be formed by treatment with aqueous acid compositions that comprise water and:    (A) a component of fluorometallate anions, each of said anions consisting of (i) at least four fluorine atoms, (ii) at least one atom of an element selected from the group consisting of titanium, zirconium, hafnium, silicon, aluminum, and boron, and, optionally, (iii) ionizable hydrogen atoms, and, optionally, (iv) one or more oxygen atoms;    (B) a component of divalent or tetravalent cations of elements selected from the group consisting of cobalt, magnesium, manganese, zinc, nickel, tin, copper, zirconium, iron, and strontium;    (C) a component of phosphorus-containing inorganic oxyanions and/or phosphonate anions;    (D) a component of water-soluble and/or -dispersible organic polymers and/or polymer-forming resins; and    (E) acidity; and, optionally, one or both of the following components    (F) a dissolved oxidizing agent, preferably a peroxy compound, more preferably hydrogen peroxide; and, optionally,    (G) a component selected from dissolved or dispersed complexes stabilized against settling, said complexes resulting from reaction between (i) substances that, before reaction, could be part of component (A) and (ii) one or more materials selected from the group consisting of metallic and metalloid elements and the oxides, hydroxides, and carbonates of these metallic or metalloid elements, to produce a reaction product other than any which could be part of component (B).
In the working examples of the '632 patent, only inorganic phosphates were used as component (C). Compositions of this type have enjoyed considerable commercial success, especially on predominantly ferriferous surfaces. However, it has recently been found that when compositions according to the most preferred examples of the '632 patent are used on predominantly zinciferous surfaces and the conversion coating formed thereby is subsequently painted, the adhesion of the paint to the coating after subsequent bending is sometimes less than is desired. Accordingly, one object of this invention is to provide compositions and processes that maintain the benefits of those taught in the '632 patent, particularly avoiding any substantial use of hexavalent chromium and other materials such as ferricyanide that have been identified as environmentally damaging, while increasing the paint adhesion to the coating in regions subjected to bending. Other alternative and/or concurrent objects will become apparent from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight; the term “polymer” includes “oligomer”, “homopolymer”, “terpolymer”, and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description and/or as produced by reaction in situ by reactions noted in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole (any counterions thus implicitly specified should preferably be selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to the objects of the invention); the term “paint” and its grammatical variations includes all more specialized terms such as “lacquer”, “varnish”, “shellac”, “enamel”, “vitreous enamel”, “self-crosslinking protective coatings”, “coating resins”, “primer”, “top coat”, “color coat”, etc; and the term “mole” and its grammatical variations means “gram mole” and its corresponding grammatical variations and may be applied to elemental, ionic, hypothetical, unstable, and any other chemical species defined by the number and type of atoms present in the species, as well as to compounds with well defined molecules.